Atomic absorption spectroanalytical instrument control system

ABSTRACT

A sensor responsive to hydroxyl ions is provided to monitor the flame at the burner in an atomic absorption spectroanalysis system. The sensor in the disclosed embodiment includes an S4 photodiode and a filter to provide a sensitivity over the wavelength band of 3000-3900 A. The output of the photodiode is applied via an FET amplifier stage and an emitter follower stage to burner control circuitry. The burner control circuitry initiates flame via a pilot line, and if hydroxyl ions are not sensed within a predetermined time, the system is automatically shut down. After flame is established, an interlock permits switching from air to nitrous oxide and on failure to sense hydroxyl ions, the nitrous oxide, the fuel and then the air are turned off in sequence in response to the output of the sensor.

United States Patent [72] Inventor Stanley B. Smith, Jr.

Lexington, Mass. [21] Appl. No 831,615 [22] Filed June 9,1969 [45]Patented June 8,1971 [73] Assignee Instrumentation Laboratory, lnc.

Lexington, Mass.

[54] ATOMIC ABSORPTION SPECTROANALYTICAL INSTRUMENT CONTROL SYSTEM 13Claims, 3 Drawing Figs.

[52] U.S. Cl. 431/79, 356/87 [51] Int. Cl F23n 5/08 [50] Field of Search431/4, 79; 356/87, 187

[56] References Cited UNITED STATES PATENTS 3,080,708 3/1963 Carr431/79X Primary Examiner-Edward G. Favors Alt0rneyWillis M. ErtmanABSTRACT: A sensor responsive to hydroxyl ions is provided to monitorthe flame at the burner in an atomic absorption spectroanalysis system.The sensor in the disclosed embodiment includes an S4 photodiode and afilter to provide a sensitivity over the wavelength band of 3000-3900 A.The output of the photodiode is applied via an FET amplifier stage andan emitter follower stage to burner control circuitry. The burnercontrol circuitry initiates flame via a pilot line, and if hydroxyl ionsare not sensed within a predetermined time, the system is automaticallyshut down. After flame is established, an interlock permits switchingfrom air to nitrous oxide and on failure to sense hydroxyl ions, thenitrous oxide, the fuel and then the air are turned off in sequence inresponse to the output of the sensor.

PATENIEDJUN elsn 3583844 'SHEET '1 OF 2 v FIGI ogre-era? RANGE I F 2HAND E MISS/0N I I 2000 000 4000 5000 a 6000 l PATENTED JUN a m SHEET 2UP 2 ATOMIC ABSORPTION SPECTROANALYTICAL INSTRUMENT CONTROL SYSTEMSUMMARY OF INVENTION This invention relates to atomic absorptionspectroscopy and more particularly to control arrangements for burnersused in such spectroanalytical systems.

In atomic absorption spectroscopy a mixture of combustible fuel andatomized solution containing material to be analyzed are fed into aburner where the material to be analyzed is energized for analysispurposes. Several different fuel oxidant mixtures are used in atomicabsorption analysis, including acetylene-air, acetylene-nitrous oxide,hydrogen-air and hydrogen-argon mixtures. Hydrogen-air andhydrogen-argon flames are particularly useful for some elementaldeterminations, including those elements analyzed in the shortultraviolet ray region near 2000 A, such as lead, iron, arsenic and tin.Loss of flame can create hazardous conditions, therefore it is an objectof this invention to provide a comprehensive burner control system whichmonitors the presence of a nonnal flame at the burner and which willautomatically turn off the fuel in the event of abnormal condition suchas loss of flame, for example due to abnormal flashback or flashout.

Another object of this invention is to provide a control system whichwill supervise the proper operation of an atomic absorption burnersystem that uses a variety of fuels.

Another object of the invention is to provide a novel and improvedatomic absorption burner control system that supervises the properoperation of the burner system when using hydrogen as a fuel as well asutilizing acetylene as a fuel.

Another object of the invention is to provide a novel and improvedatomic absorption burner control system which will automaticallysequence the fuels in desired operation and prevent or terminateignition if there is insufficient air or fuel pressure or if the burnerfails to ignite within a predetermined period of time.

In accordance with the invention there is provided an atomic absorptionburner control system which includes a sensor responsive to hydroxylions (free radicals) in the flame for controlling the operation of theburner system. In particular embodiments the flame sensor includes aphotosensor that responds to radiation including the range of emissionbands of hydroxyl free radicals. Those emission bands are in the orderof 2600 A, 2800 A, 3200 A and 3500 A and the photosensor has an upperlimit of response below 4500 A. In a particular embodiment a photocelland filter combination provide a photosensor that has a band widthresponse that includes the wavelengths 30003900 A.

In a particular embodiment the atomic absorption burner control systemincludes a nebulizer into which the sample to be analyzed is aspiratedby air which is used as an oxidant, a premixing chamber in which thenebulized sample and fuel are mixed and a burner head structure to whichthe oxidant-fuelsample mixture is supplied. This burner head structureis aligned with the optical path of the atomic absorption equipment andan igniter structure that includes a pilot fuel line and igniterelectrode is arranged to automatically ignite the main flame underoperator control. The sensor is mounted in alignment with the opticalpath for sensing the presence of flame at the main burner. The controlsystem sequences ignition and transfer between oxidants in a coordinatedmanner. Shutdown of the burner system occurs automatically in the eventof flame failure or loss of fuel or oxidant pressure.

The invention provides a comprehensive supervision system for a burnersystem in atomic absorption spectroanalytical apparatus which permitssupervised use of a variety of fuels. Other objects, features andadvantages of the invention will be seen as the following description ofa particular embodiment progresses, in conjunction with the drawings, inwhich:

FIG. I is a diagrammatic view of an atomic absorption burner controlsystem constructed in accordance with the invention;

FIG. 2 is a Colthup chart representation of the band emission ofdifferent ions in flames employed in atomic absorption spectroscopy; and

FIG. 3 is a schematic diagram of control circuitry employed in theapparatus shown in FIG. 1.

DESCRIPTION OF PARTICULAR EMBODIMENT With reference to FIG. 1, theatomic absorption burner system includes a premix chamber 10 on which ismounted a burner head 12 that is connected to the premix chamber 10 bymeans of conduit 14. The configuration of the burner head changes as afunction of the fuel employed, for example a particular burner head foruse with air as an oxidant employs an exit orifice 10 centimeters inlength having one or more slots; while a burner head for use withnitrous oxide as an oxidant has a single slot 5 centimeters in length. Aswitch 16 on conduit I4 is closed when the nitrous oxide burner head isin proper position on conduit 14. At the end of the premix chamberremote from the burner head 12 is secured an end cap that supports anebulizer structure generally indicated at 18. A capillary tube 20extends from nebulizer structure to the source of liquid sample to beanalyzed. This sample is aspirated through nebulizer 18 into premixchamber 10 by oxidant, for example air being supplied over line 22 ascontrolled by solenoid operated valve 24 and nitrous oxide beingsupplied over line 26 as controlled by solenoid operated valve 28.Switches 30, 32, respectively, are closed if pressures in the supplylines are within preestablished limits. A fuel input line 34 controlledby solenoid operated valve 36 applies fuel through metering orifice 38and manifold 40 to chamber 10. Switch 42 is closed if fuel pressure iswithin preestablished limits. Oxidant is also supplied to manifold 40via metering orifice 44.

Extending from manifold 40 is a pilot line 46 that has interposed in ita control valve 48 and has its outlet 50 (0.040 inch l.D.) spacedseveral inches from burner head 12. An igniter rod electrode 52,disposed in proximity to the outlet 50 of the pilot fuel line, isconnected to a blocking oscillator-induction coil circuit 54 which iscontrolled by circuit 56.

Supported above the burner head is a detector unit 58 which includes atype TS-433E photodiode 60 and a filter element 61 (Coming Filter No.7-54). The photodiode 60 has a sensitivity over the range 3000-5500 Awhile the filter 61 passes wavelengths in the range of 2400-3900 A. Thusthis filter and photodiode combination define a system wavelengthsensitivity in the range of 3000-3900 A. The detector 58 is mounted atan angle of 45 to the optical axis 62 of the atomic absorption system,the detector being diagrammatically shown in FIG. 1 1, from its actualsystem position in the preferred embodiment for clarity. The output ofdiode 60 is applied through two amplifier stages, the first stageincluding a field effect transistor 63 and the second stage including anNPN transistor 64, which produces an output signal for application tocontrol circuitry shown in FIG. 3.

With reference to FIG. 2, wavelengths emitted by various molecules inflames are indicated. Thus C radicals radiate in a band 65 at about 3400A and in the so-called Swan bands 66 in the green section of thespectrum; CI-l radicals radiate in a band 67 at about 4300 A; and OHradicals radiate in a series of narrow bands 68 near 3000 A. Whileacetylene produces C radicals, these radicals are not present wherehydrogen is used as the fuel. The OH radicals are produced byacetylene-air, acetylene-nitrous oxide, fuel mixtures as well ashydrogen-air and hydrogen-argon mixtures and other hydro-carbon fuelssuch as propane. The photosensor 58 has a response in range 69 and isconstrained to sense wavelengths produced by the OH radicals and toexclude sensing the C radicals. It will be obvious that otherphotosensor configurations, such as those employing interferencefilters, for example, could also be employed.

A schematic diagram of the control circuitry employed in this system isshown in FIG. 3. That circuitry controls pilot solenoid 48, fuelsolenoid 36, air solenoid 24, nitrous oxide solenoid 28 and ignitercircuitry 56 as a function of signals from sensor 58 and switches 16,30, 32 and 42 and controls which include a FLAME ON control button 70which applies a negative 12 volt signal on line 72 when the button 70 isdepressed; a FLAME OFF button 74; an air flush control 76; nitrous oxideON control switch 78 and nitrous oxide OFF control switch 80.

The circuitry further includes a main control flip-flop 82 that includestransistors 84 and 86; an oxidant control flip-flop 90 that includestransistors 92 and 94; transistor switch circuits 96, 98, 100 and 102that control solenoids 24, 28, 36 and 48 respectively and which arecontrolled in turn by transistor circuits 104, 106, 108 and 110respectively; transistor 112 which responds to the output of sensor 58;and transistors 114, 116 and 1 18 which provide sequencing control.

When the system is turned on and no flame is present at burner 12,transistors 86 and 94 are in nonconductive condition. If the fuel andoxidant pressures are above the minimum, the minimum for switch 42 beingp.s.i., for switches 30 and 32 being 8 p.s.i., 12 volt signals areapplied through those switches to the circuitry. Upon depression ofFLAME ON button 70, a negative transition is applied via diode 120, thevoltage network consisting of resistors 122 and 124 and diode 126 to thebase electrode of transistor 86 to turn that transistor on. Theresulting change in condition of transistor 86 turns output transistor128 on and a negative transition is applied via resistor 130 and diode132 to place transistor 118 in conducting condition. The resultingtransition applied via resistor 134 to gating transistor 106 which isconditioned by the conducting output transistor 136 to turn ontransistor 106 and transistor 98 to energize solenoid coil 24, providingan air flow through the mixing chamber and the burner head 12.

The turn on of transistor 118 also applies a transition via diode 138 tothe voltage divider network of resistors 140 and 142 to turn ontransistor 108 which in turn turns on transistor 100 and operates thefuel control solenoid 36 to supply fuel to the mixing chamber 10.

The transition resulting from transistor 112 being placed in conductionis also applied to the voltage divider network consisting of resistors144 and 146 to turn off transistor 114. J unction 148 then goes to +12volts potential and timing capacitor 150 commences to charge. Thetransition at junction 148 is applied through resistor 152 to diode 154to turn on transistor 116. A positive going output transition is appliedvia resistor and diode 138 to transistor 84. That transition has noeffect, however, as transistor 84 is in nonconducting condition.

Control transistor 112 is in conducting condition when there is no flamepresent. In that condition the cathode of diode 156 is at ground. At thesame time that driver transistor 128 is turned on, its complementarydriver transistor 158 on the other side of the control flip-flop 82 isturned off and the cathode of diode 160 also goes to ground allowingjunction 162 to rise to ground potential. The resulting transition isapplied via voltage divider network including resistors 164 and 166 toturn on transistor 110 which in turn causes transistor 102 to conductand energize pilot solenoid 48. The transition at junction 162 is alsoapplied via voltage divider network of resistors 168 and 170 and timedelay circuit including resistor 172 and capacitor 174 to turn ontransistor 176 of control circuit 56 after a short time delay toenergize the igniter control 54 which includes a blocking oscillator andan ignition coil to apply a spark periodically between igniter electrode52 and the tip of pilot line 50 (which is the negative electrode) toignite fuel oxidant mixture flowing through the pilot line orifice 50.

The pilot jet flame in turn ignites the main burner flame and that flameis sensed by the detector 58 so that the photocell 60 produces an outputwhich is amplified by an FET amplifier stage 178 and an emitter followertransistor stage 180 to produce a positive transition which turns offtransistor 112. With the turn off of transistor 112, the voltage atjunction 162 falls, transistors 110 and 176 cease conducting,terminating ignition and pilot fuel flow. A transition is also appliedvia the voltage network including resistors 182 and 184 and diode 186 tohold transistor 114 in nonconducting condition, thus overriding theeffect of the charging of capacitor 150. A similar transition is appliedthrough the voltage divider network of resistors 188 and 190, diode 192to hold transistor 116 in conducting condition. Should there be anintermittent flame condition so that transistor 112 returns toconduction, a transition is immediately applied via diode 186 to turntransistor 114 on and transistor 116 off and that transition throughresistor 194 and diode 196 turns transistor 84 on and resets the controlflip-flop 82.

Assuming that flame is established and remains established, the systemis now operating on a fuel air mixture. If it is desired to transferfrom air as the oxidant to nitrous oxide,

switch 78 is depressed which completes a circuit from the junction ofdiodes 198 and 200 through switch 16 (closed by the proper burner head)to the oxidant control flip-flop 90 to turn on transistor 94 and itsoutput transistor 202. The change in state of flip-flop 90 causestransistor 106 to cease conducting and transistors 104 and 96 to conductand energize the nitrous oxide control solenoid 28. It will be notedthat diode 200 is connected to nitrous oxide pressure switch 32 anddiode 202 is connected to the collector of flame detector controltransistor 112. The l2 volt signal required to operate control flip-flop90 to turn on transistor 94 will not be available unless the nitrousoxide pressure switch 32 is closed and flame is being detected at theburner head by sensor 58. When those conditions are present, the systemmay be switched from air to nitrous oxide by depression of switch 76.Depression of switch applies a 1 2 volt signal to the other input ofcontrol flip-flop to cause transistor 98 to conduct and transistor 96 tocease conducting, immediately switching from nitrous oxide to air.

Should either switch 30 or 42 open, due to air pressure or acetylenepressure drop, respectively, the voltage divider network of resistors204 and 206 will produce a transition atjunction 108 which will becoupled by diode 210 to turn transistor 86 off and by diode 212 to forcecontrol flip-flop 90 to the "air" mode, if it was not there already.Should the nitrous oxide pressure sensing switch 32 open, a voltagetransition will be coupled by the divider network of resistors 214 and216 and diode 218 to switch the control flip-flop 90 to the air" modeand terminate the flow of nitrous oxide if the system had been in thatmode. The control flip-flop 82 is also reset on depression of the FLAMEOFF button 74.

When control flip-flop 82 is reset, transistors 84 and 158 are turnedon. Transistors 86 and 128 are turned off and capacitor 220 connectedbetween the base and collector electrodes of transistor 118 starts tocharge. Transistor 108 is turned off after a 7 second delay terminatingfuel flow and transistor 106 is turned off after a 10 second delay, thusterminating the flow of air to the burner system and completing a shutdown of the burner in safe condition.

While a particular embodiment of the invention has been shown anddescribed, various modifications thereof will be apparent to thoseskilled in the art and therefore it is not intended that the inventionbe limited to the disclosed embodiment or to details thereof anddepartures may be made therefrom within the spirit and scope of theinvention as defined in the claims.

What 1 claim is:

1. In an atomic absorption spectroanalysis system comprising meansdefining an optical axis, a burner disposed below said optical axis forproducing a flame to energize a sample to be analyzed so that saidenergized sample passes across said opticalaxis to modify radiation insaid optical axis in an atomic absorption analysis, and means to supplya fuel oxidant mixture to said burner to produce said flame, theimprovement of a sensor responsive to hydroxyl ions for monitoring thepresence of flame at said burner.

2. The apparatus as claimed in claim 1 wherein said sensor is aphotosensor responsive to emission from hydroxyl ions in the flame.

3. The apparatus as claimed in claim 2 wherein said photosensor has aband width of response, the upper limit of which is below 4500 A.

4. The apparatus as claimed in claim 3 wherein said photosensor has abandwidth of response that includes the wavelength of 3000-3900 A.

5. An atomic absorption spectroanalysis system comprising a burner forproducing a flame to energize a sample to be analyzed, a chamber forsupplying a fuel-oxidant-sample mixture to said burner, a sensorresponsive to hydroxyl ions for monitoring the presence of flame at saidburner,

a first control for controlling the supply of fuel to said chamber, asecond control for controlling the supply of oxidant to said chamber andcircuitry responsive to said sensor for controlling said first andsecond controls 6. The apparatus as claimed in claim 5 wherein saidcircuitry further includes an igniter for igniting a flame, a flameinitiating control, circuitry responsive to said flame initiatingcontrol for operating said first and second controls and said igniter insequence and circuitry responsive to said sensor for terminating flow ofoxidant and fuel to said chamber unless said sensor produces an outputsignal indicative of flame within a predetermined interval.

7. The apparatus as claimed in claim 6 wherein said sensor responsivecircuitry operates said first control to terminate flow of fuel to saidchamber prior to operation of said second control to terminate flow ofoxidant to said chamber in the absence of said output signal from saidsensor.

8. The apparatus as claimed in claim 5 and further including means forsupplying first and second oxidants to said chamber,

each said oxidant employing a different burner unit, said second controlcontrolling the supply of one of said oxidants and a third controlcontrolling the supply of said second oxidant and an interlockresponsive to a burner unit for controlling the operation of said thirdcontrol.

9. The apparatus as claimed in claim 8 and further including pressureresponsive control for sensing the supply pressure of said secondoxidant and circuitry responsive to a low supply pressure of said secondoxidant for operating said third control to terminate the flow of saidsecond oxidant to said chamber and for automatically operating saidsecond control to initiate the flow of said first oxidant to saidchamber.

10. The apparatus as claimed in claim 9 wherein said circuitry includesa first bistate device for controlling burner operation and a secondbistate device for controlling the supply ofoxidant to said chamber.

11. The apparatus as claimed in claim 10 wherein said sensor is aphotosensor responsive to emission from hydroxyl ions in the flame.

12. The apparatus as claimed in claim 11 wherein said photosensorincludes a photocell and a filter arranged to have a response bandwidthwhich includes the wavelengths of 3000-3900 A.

13. The apparatus as claimed in claim 11 wherein said photosensor is aphotodiode and has an output terminal connected to an amplifier circuitthat includes a field effect transistor for generating an output signalfor control of said first, second and third controls as a function ofthe flame condition at said burner.

2. The apparatus as claimed in claim 1 wherein said sensor is aphotosensor responsive to emission from hydroxyl ions in the flame. 3.The apparatus as claimed in claim 2 wherein said photosensor has a bandwidth of response, the upper limit of which is below 4500 A.
 4. Theapparatus as claimed in claim 3 wherein said photosensor has a bandwidthof response that includes the wavelength of 3000-3900 A.
 5. An atomicabsorption spectroanalysis system comprising a burner for producing aflame to energize a sample to be analyzed, a chamber for supplying afuel-oxidant-sample mixture to said burner, a sensor responsive tohydroxyl ions for monitoring the presence of flame at said burner, afirst control for controlling the supply of fuel to said chamber, asecond control for controlling the supply of oxidant to said chamber andcircuitry responsive to said sensor for controlling said first andsecond controls.
 6. The apparatus as claimed in claim 5 wherein saidcircuitry further includes an igniter for igniting a flame, a flameinitiating control, circuitry responsive to said flame initiatingcontrol for operating said first and second controls and said igniter insequence and circuitry responsive to said sensor for terminating flow ofoxidant and fuel to said chamber unless said sensor produces an outputsignal indicative of flame within a predetermined interval.
 7. Theapparatus as claimed in claim 6 wherein said sensor responsive circuitryoperates said first control to terminate flow of fuel to said chamberprior to operation of said second control to terminate flow of oxidantto said chamber in the absence of said output signal from said sensor.8. The apparatus as claimed in claim 5 and further including means forsupplying first and second oxidants to said chamBer, each said oxidantemploying a different burner unit, said second control controlling thesupply of one of said oxidants and a third control controlling thesupply of said second oxidant and an interlock responsive to a burnerunit for controlling the operation of said third control.
 9. Theapparatus as claimed in claim 8 and further including pressureresponsive control for sensing the supply pressure of said secondoxidant and circuitry responsive to a low supply pressure of said secondoxidant for operating said third control to terminate the flow of saidsecond oxidant to said chamber and for automatically operating saidsecond control to initiate the flow of said first oxidant to saidchamber.
 10. The apparatus as claimed in claim 9 wherein said circuitryincludes a first bistate device for controlling burner operation and asecond bistate device for controlling the supply of oxidant to saidchamber.
 11. The apparatus as claimed in claim 10 wherein said sensor isa photosensor responsive to emission from hydroxyl ions in the flame.12. The apparatus as claimed in claim 11 wherein said photosensorincludes a photocell and a filter arranged to have a response bandwidthwhich includes the wavelengths of 3000-3900 A.
 13. The apparatus asclaimed in claim 11 wherein said photosensor is a photodiode and has anoutput terminal connected to an amplifier circuit that includes a fieldeffect transistor for generating an output signal for control of saidfirst, second and third controls as a function of the flame condition atsaid burner.